Modem aircraft are routinely provided with environmental control systems (ECS). In civilian aircraft such as airliners, these provide air supply, thermal control and cabin pressurization for the crew and passengers. In military aircraft, the ECS is responsible for cooling of avionics, pressurization and temperature control of the cabin and the crew's suits, windshield defrosting, and sundry other tasks. In both cases, the ECS normally includes tubes and pipework through which fluids such as air are moved.
In certain conditions, the air pipes of an ECS can suffer from icing on their inner surface; that is, the pipes become so cold that any moisture in the air freezes out onto the inner surface of the pipes. This is undesirable, as the ice can break off when the pipe heats up, and the fragments of ice can become entrained in the air flowing through the pipe and cause problems downstream. In addition, the build-up of ice reduces the inner diameter of the pipe, and may reduce the flow of air below acceptable limits. It is thus desirable to maintain the air pipes at temperatures where this problem, and other temperature-related critical conditions, can be avoided.